Day 20 of 100: Error Handling — try/catch, finally, Custom Errors

Day 20 of 100 — 100 Days of Playwright with TypeScript Challenge. Phase: JavaScript Fundamentals. JavaScript (Days 1-33) → TypeScript (Days 34-63) → Playwright (Days 64-96) → AI Framework Capstone (Days 97-100).

🎭 Want to master this with real projects? Join the Playwright Automation Mastery course at The Testing Academy.

---

Tests fail. Networks drop. Good error handling separates clear failures from mystery stack traces.

try / catch / finally

async function fetchUser(id) {
  try {
    const res = await fakeApi(id);
    return res;
  } catch (err) {
    console.error('fetchUser failed: ' + err.message);
    throw err;            // re-throw — do not swallow!
  } finally {
    console.log('cleanup runs no matter what');
  }
}

Custom Error Classes

class TestDataError extends Error {
  constructor(message, entity) {
    super(message);
    this.name = 'TestDataError';
    this.entity = entity;
  }
}

function createUser(data) {
  if (!data.email) {
    throw new TestDataError('email required', 'user');
  }
}

try { createUser({}); }
catch (e) {
  if (e instanceof TestDataError) console.log('data problem: ' + e.message);
  else throw e;   // unknown errors bubble up
}

Rules for Test Code

• Never catch-and-ignore — silent failures are worse than crashes
• Catch only where you can act (retry, cleanup, better message)
• Let Playwright assertions throw naturally — the runner formats them
• finally is for cleanup: delete test data, close connections

Tomorrow (Day 21): JSON — parsing and validating API payloads.